The present invention relates to a pressure control valve for fuel injection pumps.
Pressure control valves of the foregoing type include a cylinder with a piston displaceable therein and biased by a spring on one side thereof. A throttle is provided in the spring chamber of the cylinder through which leaking fuel is discharged from the cylinder to the fuel supply tank. In such pressure control valves an annular gap is provided at the entire periphery of the piston and extends over its entire length. The piston of the pressure control valve is also guided within the cylinder of the valve with a relatively great gap, which is disadvantageous because the position of the piston in the radial direction is not fixed so that the piston can take any possible positions relative to the wall of the cylinder in the radial direction. This, however, means that it is not ensured that the width of the annular gap will be maintained constant. With the one-side position of the piston, partially the gap of a substantially greater width will result so that in this position of the piston the amount of fuel flowing through the gap can vary for example by 2.5 as compared to that for the concentrically positioned piston.
In the conventional device the annular gap, in connection with the control pressure chamber and the spring chamber formed in the cylinder, and together with an aperture-shaped throttle serve to compensate for the influence of the viscosity, changing with temperature fluctuations, on the pressure of the fluid being controlled. The amount of fuel flowing through the annular gap directly depends on the viscosity of the flowing medium while the amount of fuel flowing through the throttle does not depend on the viscosity. A pressure in the spring chamber of the cylinder results from the difference of both amounts of flowing fuel, which pressure is dependent upon the viscosity and exerts on the piston a compensating additional force. When now the amount of fuel flowing through the annular gap changes in dependence on the position of the piston the influence of the errors caused by temperature fluctuation on the pressure being controlled is rather considerable.